Paramecium (26 stocks of P. primaurelia and 10 stocks of P. triaurelia) have been collected from diverse North American locations. The tolerance of each of these stocks to mercury, cadmium, copper, nickel, zinc, cobalt, chromium and manganese (listed in order from most toxic to least toxic) has been determined. Analysis of variance has revealed highly significant differences among stocks in tolerance to mercury, copper, nickel and cobalt. The largest within species differences occur in copper tolerance. The proposed work will focus on genetic analysis of the differences. A quantitative approach will be used for the seven metals excluding copper and for copper tolerance differences the study will attempt to identify alleles at specific loci. The metal specificity of tolerance will be studied.